1. Field of the Invention
The present invention relates to a charged particle beam lithography system, a lithography method using a charged particle beam, a method of controlling a charged particle beam, and a method of manufacturing a semiconductor device.
2. Related Background Art
A charged particle beam lithography system has the function capable of forming a pattern of high resolution since it can write patterns with resolution of a wavelength level of an electron (ion) of which wavelength is shorter than light. On the other hand, different from a mask lithography method using light exposure, the charged particle beam lithography system directly form a complete pattern with small divided pattern beams, so that the apparatus has a drawback such that it takes long time for lithography.
The apparatus, however, has a feature that it can form a fine pattern of high precision and is developed as a technique following a lithography technique of light lithography method or an effective tool for manufacturing a semiconductor integrated circuit device for multi-product small-quantity production such as an ASIC (Application Specific Integrated Circuit). Examples of a method of forming a pattern directly with a charged particle beam such as an electron beam include a method of forming a pattern by scanning the whole face of a wafer while turning on or off a small circular spot beam and a VSB lithography method of forming a pattern with an electron beam which passes through a stencil aperture (for example, H. Sunaoshi et al, Jpn. J. Appl. Phys. Vol. 34 (1995), pp. 6679-6683, Part 1, No. 128, December 1995). As a lithography method obtained by further developing the VSB lithography method, an electron beam lithography technique of a one-shot exposure type of forming a pattern at high speed by preparing a stencil so that one block is constructed by repetitive patterns and performing selective exposure has been also developed.
An electron beam lithography system for the VSB lithography method disclosed by H. Sunaoshi et al. comprises an electron-optical system constructed by an electromagnetic lens and an electrostatic deflector. Consequently, the apparatus is required to comprise the configuration in full consideration of total optical characteristics of the lenses and deflector, mechanical assembly precision, influence of contamination and the like. To improve the beam resolution, a method of irradiating a resist on a wafer with highly accelerated electron beams is employed. It causes a proximity effect phenomenon such that back scattered electrons are generated in various multilayer thin films formed between the resist and the wafer by the irradiated electron beams and move upward in the resist. This proximity effect causes a blur in a pattern or deterioration in resolution. Therefore, a control for correcting the proximity effect is necessary and not only the electron optical system but also a large-scaled control system have to be constructed. As a result, the system becomes complicated, which might induce a further trouble, and a problem of deterioration in precision occurs. Since a highly-accelerated electron beam is used, damage on the surface of the wafer is also concerned.
To solve the problems of the VSB method using the highly-accelerated voltage charged particle beam, an electron beam lithography method of an aperture type using
To solve the problems of the VSB method using the highly-accelerated voltage charged particle beam, an electron beam lithography method of an aperture type using a low-accelerated voltage electron beam has been proposed (for example, Japanese Patent Laid-open No. 2000-173529 and J. Vac. Sci. Technol. B14(6), 1996, 3802). An electron optical system of an electron beam lithography system disclosed in Japanese Patent Laid-open No. 2000-173529 uses an einzel lens for a demagnification projection optical system and, consequently, as shown in FIG. 1, an electron beam 67 draws trajectories which are rotation symmetric with respect to an optical axis. Consequently, the trajectory of the electron beam 67 is deflected with the same deflection sensitivity by a pre-main deflector 95′, a main deflector 95, a pre sub-deflector 93′, and a sub-deflector 93. A deflection aberration occurs also rotation symmetrically with respect to the optical axis.
However, in a demagnification projection optical system of the electron beam lithography system disclosed in Japanese Patent Laid-open No. 2000-173529, crossovers 98 and 99 of high current density are formed below a cell aperture 19. In the projection optical system, rotation-symmetrical electrostatic lenses (einzel lenses) 64 and 66 are employed in a deceleration-type focus mode, so that the electron beam 67 is decelerated in the lenses. Due to the two phenomena, the electron beam lithography system disclosed in Japanese Patent Laid-open No. 2000-173529 has problems in that a beam blur due to chromatic aberration and space-charge effect (particularly, Boersch effect) occurs, a cell aperture image is blurred on a wafer 14 and, as a result, a lithography characteristic degrades.
To solve the problems in the charged particle beam lithography method of an aperture type using a charged particle beam of a low-accelerated voltage, a lithography method in which a demagnification projection optical system is constructed with multi-fold multipole lenses has been proposed (Japanese Patent Laid-open Nos. 2001-93825, 2002-50567, 2002-93357, and 2002-216690). FIG. 2 shows an electron beam lithography system disclosed in Japanese Patent Laid-open No. 2002-50567. In the charged particle beam lithography system 100 shown in FIG. 2, a demagnification projection optical system in an electron optical system is constructed with fourfold multipole lenses. When an electron beam is used as a charged particle beam, an electron beam 8 accelerated from an electron gun 11 falls on a first aperture 13 having a rectangular or circular shape. The electron beam 8 passes through the first aperture 13 and travels toward the cell aperture 19 in which a plurality of one-shot exposure cell patterns are arranged. The electron beam 8 is shaped by an illumination lens 15 so as to have a beam radius which is sufficiently large for an arbitrary cell pattern and does not interfere with adjacent cell patterns. The illumination lens 15 is constructed by two electrostatic lenses 15a and 15b (einzel lenses) and used by applying a negative voltage to a center electrode. The electron beam 8 from the second illumination lens 15b is deflected by a first shaping deflector 17 so that a target cell pattern in the cell aperture 19 is selected. After the electron beam 8 passes through the cell aperture 19, a cell aperture image is deflected back to the optical axis by a second shaping deflector 21. The electron beam 8 which has passed through the first shaping deflector 17 and cell aperture 19 starts as a cell pattern beam from the cell aperture 19 as a starting point and enters a multipole lens 23 in a state where the electron beam 8 is deflected back to the optical axis by the second shaping deflector 21. The multipole lens 23 is constructed with fourfold electrostatic lenses Q1 through Q4 to generate quadrupole fields (multipole lens fields) by using an octopole electrode.
It is assumed that the optical axis is a Z axis, two planes which are orthogonal to each other on the Z axis are an X plane and a Y plane, the trajectory of an electron beam on the X plane is an X trajectory, and the trajectory of an electron beam on the Y plane is a Y trajectory. A voltage is applied to the fourfold lenses Q1 through Q4 of the multipole lens 23 so that electric fields in two directions of the X and Y directions become a divergence electric field, a divergence electric field, a convergence electric field, and a divergence electric field in the X direction and a convergence electric field, a convergence electric field, a divergence electric field, and a convergence electric field in the Y direction, respectively. Shield electrodes 36 or 39 as ground electrodes are disposed in the vicinities of both sides of the multipole lens 23, first shaping deflector 17, second shaping deflector 21 and pre-main deflectors 25a and 25b in the optical axis direction. The shield electrode 36 between the first lens and second lens of the multipole lens 23 and the shield electrode 39 just above the pre-main deflector 25 serve as apertures 38 and 41, respectively. By detecting a beam current in the apertures 38 and 41, beam alignment between the illumination lens 15, first shaping deflector 17, second shaping deflector 21 and the lenses Q1 and Q2 of the multipole lens 23 is performed. FIG. 3 shows the trajectories of the electron beam 8 from the cell aperture 19 to the wafer 14. The electron beams 8 passes through the trajectories 8X and 8Y which are different from each other in the X and Y directions by the influences of each of the electric fields generated by the lenses Q1 through Q4 of the multipole lens 23 and are converged on the wafer 14 without forming a region of high electron density. With respect to the wafer 14 mounted on the XY stage, the location of a main field is controlled under a main deflection control carried out by superimposing deflected electric fields on the electric fields generated by pre-main deflector 25a and the multipole lenses 23 Q3 and Q4 so as to serve as deflectors, and the location of a sub field is controlled with a sub deflector 31 while referring to the position of an XY stage (not shown). A deflection aberration which occurs on the wafer 14 is controlled so as to be minimized by adjusting the deflection voltage ratio in the main deflection control under which the multipole lenses 23 Q3 and Q4 are controlled as deflectors by superimposing deflected electric fields on the electric field generated by the pre-main deflector 25 between the multipole lens 23 Q2 and Q3 and the electric fields generated by the multipole lens 23 Q3 and Q4. The inner radius of each of the multipole lenses 23 Q3 and Q4 on which the deflected electric fields are superimposed is designed to be larger than that of the quadruple lenses Q1 and Q2 (refer to FIG. 2). By the designing, the deflection aberration can be reduced. For example, as shown in FIG. 4, the deflection aberration is minimized through adjustment of the deflection voltage ratio by deflecting the electron beam in the X direction with the pre-main deflector 25a, main deflector 23 (Q3, 27) and sub deflector 31 (a trajectory 48X of the deflected beam in the X direction) and by deflecting the electron beam in the Y direction only with the main deflector 23 (Q3, 27) and the sub deflector 31 (a trajectory 48Y of the deflected beam in the Y direction).
However, in an optical system using the multipole lens 23 for a demagnification projection, the electron beam 8 passes through the trajectories which are largely asymmetric with respect to the optical axis, and the aberration characteristic in the X direction and that in the Y direction are largely different from each other. As a result, a cell aperture image is terribly blurred asymmetrically on the wafer 14.
On the other hand, when a multipole lens is applied to an electron optical lens and the trajectories which are largely asymmetric with respect to the optical axis are formed so as not to form a region 99 of high electron density in order to reduce a space-charge effect, a problem in that the lithography characteristic degrades occurs.
Hitherto, a method of correcting spherical aberration and chromatic aberration is use in which an einzel lens is employed for an optical system of an electron microscope for the like and a multipole lens is incorporated as a aberration corrector in a part of the optical system (for example, Japanese Patent Laid-open No. 5-234550, J. Zach and M. Haider, “Aberration correction in a low-voltage scanning microscope”, Nuclear Instruments and Methods in Physics Research (Section A), Vol. 363, No. 1, 2, pp. 316-325, 1995, and J. Zach, “Design of a high-resolution low-voltage SEM by a multipole corrector”. Optik 83, No. 1, pp. 30-40, 1989).
However, in the case of correcting the spherical aberration and chromatic aberration by an optical system constructed by an einzel lens, an aberration corrector has to be assembled separately from an image forming optical system. The method has a drawback such that, as a result, the optical length becomes longer and a blur due to the space-charge effect is worsened.